1. Field of the Invention
The present invention is generally concerned with insulative materials and more particularly with devices and methods for verifying that such materials conform to a reference material in order to detect defects or contamination of such materials. The invention further concerns a method for sorting insulative material granules to be used to manufacture power cable sheathing, for example.
2. Description of the Prior Art
Detecting defects and/or contamination of insulative materials is of primary importance in the case of synthetic insulative materials that are used to manufacture sheathing of high or very high voltage power cables, for example. The very high voltages at which these cables operate require very extensive quality control to guarantee the quality of the insulative materials used to manufacture the insulative sheathing of the cable. The smallest defect in the insulative sheathing can lead to breakdown of the insulative material and thus to irreversible deterioration of the cable.
The synthetic insulative material to be used to manufacture insulative cable sheathing is in practise shipped in the form of insulative material granules, each having a very small volume, in the order of ten cubic millimeters. The granules are heated for an extrusion operation. Various kinds of defects and/or contamination affect the granules. They comprise surface contamination, volume contamination and generalized volume defects.
Surface contamination is caused by cutting up the insulative material to produce the granules and transporting the resulting granules. It comprises mechanical fines, polymer dust, mineral contaminants and exudation from the constituents. The mechanical fines are the result of the cutting operation, which produces insulative material dust which is deposited onto the granules formed by the cutting operation. The mineral contaminants are caused by contact of the granules with external elements, such as metal or plastics material transportation containers, which cause localized deposition of plastics material or metal micro-elements on the granules.
The volume contamination is the result of the operation by which the insulative material is manufactured. It comprises conductive or insulative mineral bodies that are locally integrated into the volume of a granule. These mineral bodies are present in the furnace in which the insulator is made, for example, in the additives required to manufacture it or in the manufacturing environment. An oxidized area can also appear following prolonged contact with oxygen at a high temperature.
The volume defects are in the form of generalized defects in the chemical structure of the granule. They originate in malfunctions of the reactor in which the insulator is manufactured, for example.
Contamination and defects of the above kind in practise affect only a relatively small percentage of the granules. However, it is essential to provide a method of detecting such imperfections wherever possible and for rejecting imperfect granules if the imperfection cannot be eliminated.
Various prior art techniques are available for eliminating surface contamination on the granules. They include washing in a solvent such as methanol, for example. Eliminating surface contamination does not pose any particular problem, and the prior art provides well proven techniques for this.
Eliminating volume contamination and defects relies on optical techniques that are not entirely satisfactory. The objective of such techniques is to reject granules that have volume contamination and/or defects, given that volume imperfections of the granules cannot be corrected. Broadly speaking, they involve projecting a light beam onto each granule in order to obtain an image of each granule. Image processing is then used to detect a color difference between the color of at least a localized area of the granule of interest and a reference color. The color difference determines whether the granule has any imperfection and must therefore be rejected. This optical technique has the drawback of being relatively inefficient in the case of volume contamination and even less efficient in the case of generalized volume defects. Being based on detecting granule imperfections by chromatic analysis, the technique is inherently limited by the fact that volume contamination or defects do not necessarily cause any chromatic differentiation, or at least any differentiation detectable by optical means.
A first objective of the invention is to provide a method and device for verifying that a given insulative element conforms to a reference insulative element, in order to detect efficiently volume defects and contamination of the given insulative element, e.g. granule.
A second objective of the invention is to provide a method of sorting insulative material granules.
A final objective of the invention is to provide a power cable manufactured from insulative material granules sorted by the above method.